Tsarnaev Brothers Allegedly Planned to Originally Strike on July 4, After Watching Online Videos by Anwar al-Awlaki

The surviving suspect in the Boston bombings has told investigators that he and his brother were inspired partly by watching online videos of a radical Muslim cleric who was killed by the U.S. in 2011, a U.S. official said, offering a new glimpse of the brothers' alleged radical turn that echoes earlier terrorism cases.

Anwar al-Awlaki, a U.S. citizen living in Yemen, was killed by a U.S. drone strike. He previously played a role in the radicalization of "underwear bomber" Umar Farouk Abdulmutallab of Nigeria, who pleaded guilty to trying to blow up a U.S.-bound airliner on Christmas Day 2009.

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Unlike Mr. Abdulmutallab, who traveled to Yemen for instruction from Mr. Awlaki, investigators haven't found evidence of direct contact between the Tsarnaevs and the radical cleric, the U.S. official familiar with the suspect's statements said.

The brothers— Tamerlan Tsarnaev, 26, and Dzhokhar Tsarnaev, 19—are accused of setting off two bombs at the Boston Marathon on April 15, killing three people and injuring more than 200. Dzhokhar Tsarnaev was arrested on April 19, a Friday, and answered investigators' questions over the subsequent weekend from his hospital bed, where he was receiving treatment for injuries suffered while fleeing the police.

A gunfight with police killed Tamerlan Tsarnaev, whose body was turned over to a funeral service retained by his family on Thursday afternoon, a Massachusetts state spokesman said.

Officials have said Dzhokhar Tsarnaev stopped speaking to interrogators after being informed of his right to remain silent by a magistrate judge on April 22.

Officials have said that during interrogations the younger brother described the attack as motivated by jihadist ideology and suggested he and his brother grew radicalized online.

Also, Mr. Tsarnaev told interrogators he and his brother had considered carrying out their attack on July 4, but settled on April 15, which was celebrated as Patriots' Day in Massachusetts, the U.S. official said. Officials said Mr. Tsarnaev said the brothers built their pressure-cooker bombs in the older brother's Cambridge, Mass., apartment.

U.S. investigators are still trying to determine whether the brothers carried out their alleged terrorist plot on their own or were aided by others. The older brother traveled to Russia for six months in 2012.

On Wednesday, U.S. prosecutors charged two 19-year-old Kazakh college friends of the younger Mr. Tsarnaev— Dias Kadyrbayev and Azamat Tazhayakov—with helping hide evidence of the bombs by throwing out a backpack with fireworks inside. A third friend, Robel Phillipos, 19, was charged with lying to prosecutors. None of the suspects have entered a plea.

Kazakhstan's foreign ministry said Thursday it was cooperating with the U.S. probe and offering consular help to Mr. Kadyrbayev and Mr. Tazhayakov. Lawyers for the two said they were cooperating with authorities, and Mr. Kadyrbayev's lawyer said his client didn't know the backpack he allegedly discarded contained evidence linked to the bombing.

Mr. Phillipos's lawyer declined to comment beyond saying his client hasn't been accused of any offense related to the bombing.

If convicted, Mr. Phillipos faces up to eight years, compared with a maximum of five years for the two Kazakhs, because of a federal law increasing the penalties for those convicted of lying about a terrorism investigation.

After the capture, federal officials questioned Mr. Kadyrbayev and Mr. Tazhayakov, and subsequently began trying to deport them, saying they had violated their student visas—an allegation their lawyers contest.

Linda Cristello, a Boston immigration lawyer representing the two Kazakhs in the separate deportation proceedings, said their friendship with Mr. Tsarnaev isn't surprising because "foreign students stick together." The University of Massachusetts Dartmouth, which all three attended, has just a few Russian-speaking students.

Mr. Kadyrbayev was struggling in school, his lawyer said. He stopped going to classes last fall after his grades fell, and he wanted to return home, Ms. Cristello said.

Kazakhstan is a former Soviet republic in Central Asia that has witnessed economic growth in the past decade largely thanks to oil. Mr. Tazhayakov hails from Atyrau, an oil-industry hub in the west of Kazakhstan. His father, Amir Ismagulov, is the chairman of the board of directors at an industrial company and a politician, according to Kazakh media reports.

"Everyone who knows my son knows he never fought with anyone," Mr. Ismagulov said in an interview aired on Russian television. "He was never associated with any radicals."

Mr. Kadyrbayev is from Almaty, Kazakhstan's main city. In a TV interview last week, Mr. Kadyrbayev's father said the "Terrorista #1" license plate that appeared on his son's black BMW was a joke gift from Spanish friends. "It's a joke that has dramatically turned on them," he said.

Ahmad Nassri, 20, who played indoor soccer with Mr. Tsarnaev, said Mr. Tsarnaev owned a messy Honda Civic, but occasionally he would borrow the black BMW. "One time I called him rich when he showed up in a BMW, and he was like, 'No dude, this is my friends' car,' " Mr. Nassri said.

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